A common practice in the design and construction of rotating electromechanical systems is the use of mechanical coupling devices.
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment.
Shaft couplings are used in machinery for several purposes, the most common of which are the following:    To provide for the connection of shafts of units that are manufactured separately such as a motor (motor and gear input, gear output and load) and to provide for disconnection for repairs or alterations.    To provide for misalignment of the shafts or to introduce mechanical flexibility.    To reduce the transmission of shock loads from one shaft to another.    To alter the vibration characteristics of rotating units.
Mechanical overload may damage or interrupt the system operation.
The consequences of undetected overload in mechanical systems can be grave. Countless systems suffer overload damages at all times. Overload can occur abruptly (for example some object falling into the rotating area) or build up gradually over many months of operation without being noticed.
When overload failure occurs damages can manifest in many different ways such as local system breakage, production lines stalled for many hours or days, fire hazard etc. Economical impact may be very substantial.
Couplings do not normally allow disconnection of shafts during operation, however there are specialized torque limiting couplings which can slip or disconnect when some torque limit is exceeded. State of the art protected couplings have some disadvantages:    A slipping coupling may slip for a very long time without being noticed.    When slipping is detected maintenance people often tend to just tighten the adjustment screws as a quick fix, overriding intended protection level, therefore making this protection useless.    Disconnecting (mechanical fuse) type protection forces the replacement of the coupling unit device upon failure on top of expenses associated with fixing the major cause of overload.